1. Field of the Invention
The present invention relates to an apparatus for manufacturing a silicon single crystal by means of the "Czochralski pulling technique", and more particularly to a method for feeding granular silicon into a crucible.
2. Description of the Prior Art
An apparatus heretofore known for the manufacture of silicon single crystals by the Czochralski method includes an apparatus of the type which continuously pulls a silicon single crystal while feeding granular silicon into a crucible. The silicon single crystal is grown in an inert atmosphere under reduced pressure to avoid incorporation of impurities into the molten silicon. Accordingly, an apparatus for feeding granular silicon material comprises a material storage hopper, a feeder, a guide tube and a housing enclosing said hopper, said feeder and said guide tube. The granular silicon material is stored in the material storage hopper. A predetermined amount of said silicon material is discharged from a lower discharge port of the hopper and is fed by the feeder to the crucible through the guide tube. Japanese Examined Patent Publication No. 17537/86 discloses an example of the aforementioned.
In the "Czochralski pulling technique" wherein the granular silicon material is continuously fed into the crucible, when the granular silicon material in the hopper has been used up, the hopper is replenished with granular silicon material. When the granular silicon material is charged into the hopper, measures should be taken so that air cannot be introduced into a chamber of the apparatus for pulling a silicon single crystal. Accordingly, a sluice valve, which can cut off communication between the hopper and the chamber, and which is fit for vacuum sealing, is mounted on a position of the housing enclosing the material storage hopper. The granular silicon material is charged into the hopper after the sluice valve has been closed and the pressure inside the housing having the hopper therein has been elevated to atmospheric pressure.
However, when the sluice valve is mounted on one position of the housing, the guide tube leading the granular silicon to the crucible cannot be passed through the sluice valve. That is, the guide tube is divided into two portions before and behind the sluice valve. Accordingly, when the granular silicon is fed to the crucible, the granular silicon freely falls by its weight from the guide tube positioned before the sluice valve into the guide tube positioned behind the sluice valve. During the fall of the granular silicon, the granular silicon strikes the guide tube positioned behind the sluice valve, and either rebounds or scatters. When the granular silicon rebounds or scatters, particles of silicon attach to the surface of a seat of the sluice valve or accumulate thereon. In the case where particles of silicon are attached to the surface of the seat of the sluice valve, when fresh granular silicon is charged into the hopper, the particles of silicon are gripped between the seat and the disc of the sluice valve although the sluice valve is closed, lowering the capability of the sluice valve for vacuum sealing. In consequence, air is introduced into the chamber, and the successive growth of silicon single crystals cannot be carried out.